Here's what you can do:
- Walk to the chest behind the gallows, the chest where you found the augmentation.
- Teleport to another region.
- Teleport back to Headsman's Hill.
You will now be outside the opening to the cave where you ended your fight with Thunder. Once you get whatever you need (either from the silver chest or from fishing in the pond), you can teleport back out.
Q.E.D. (I know: it's inappropriate, but it looks nice
)